<SCRIPT LANGUAGE="JavaScript">
function CheckAll(chk){
for (i = 0; i < chk.length; i++)
chk[i].checked = true ;
}

function UnCheckAll(chk){
for (i = 0; i < chk.length; i++)
chk[i].checked = false ;
}
</script>

<form name="test" action="/">
<input name="category[]" type="checkbox" value="a">a
<input name="category[]" type="checkbox" value="b">b
<input name="category[]" type="checkbox" value="c">c
<input name="category[]" type="checkbox" value="e">d

<input name="CheckAll" type="button" value="Select All"
onclick="CheckAll(document.test['category[]'])" />
</form>